Conventionally, is has been known a head-up display apparatus that projects a display image onto a windshield or the like of a vehicle so that a virtual image of the display image can be viewed from an assumed viewing space. As a kind of such apparatus, for example, a patent literature 1 discloses a scanned-beam head-up display apparatus having a microlens array on which a display image is depicted by a scanning beam projected from a beam generator. The microlens array is made of a plurality of lenslets arrayed for expanding a laser beam toward the viewing space.
When the scanning beam is applied to the microlens array of the patent literature 1 in which the lenslets having the same shape are arrayed, a laser beam diffracted by one lenslet and a laser beam diffracted by another lenslet adjacent to the one are interfered with each other and intensified. In the case where the lenslets have the same shape, positions at which the laser beams are intensified by being interfered to each other are regularly aligned in the display image. Therefore, even when the laser beam is scanned, the position at which the laser beams are intensified by being interfered are not substantially moved. As a result, intensity distribution of the laser beams reaching the viewing space has unevenness, and thus the display image viewed by a viewer has unevenness.
In the structure disclosed by the patent literature 1, a pair of microlens arrays is arranged in an opposed manner. In the structure, a laser beam having been passed through one of the microlens arrays is diffused by the other one of the microlens, thereby reducing the unevenness of the intensity distribution caused by the interference. In the apparatus of the patent literature 1, however, plural microlens arrays are used, and it is necessary to adjust the positions of the plural microlens at high accuracy. As a result, the structure associated with the microlens arrays are necessarily complicated.